The largest list, with some half-million people on it, included suspected marathon bomber Tamerlan Tsarnaev and his mother.

But that didn't stop Tsarnaev and his brother from allegedly executing the Boston Marathon bombings.

However, Boston is not alone in its vulnerability to terrorist attacks on American soil.

"There really is a tremendous vulnerability about our city center," said terrorism analyst Tony Cooper about Dallas, Texas.

In 2009 there was a thwarted attempt to bomb a Dallas high-rise building by Maher Husein Smadi. He was on a high-level terror watch list, under constant surveillance by the FBI.

Another terrorist, Mohamed Osman Mohamud, was on the government's no-fly list.

In 2010, he planned to detonate a bomb at a Christmas tree lighting in Portland, Ore.

Neither man knew they were part of undercover sting operations. The explosives were dummies.

"Someone was assigned from the FBI to start watching them and eventually someone offered them explosives. Of course, this was someone working with law enforcement," said terrorism expert Leonard Marcus. "They actually pressed a button to detonate explosives. So they were prosecuted for a terrorist act," he said.

Both cases point to how terror watch lists can stop an attack and take terrorists off the street. But which list they are on determines the level of scrutiny and surveillance by the government.

"If they are talking to known bad guys, receiving money through routes used by terrorists, those are all clues," said Marcus. "If they are on a very high watch list it's possible to monitor their emails, listen to conversations, determine what their plans are."

But most people, some half a million, are considered low risk and placed on the Terrorist Identities Datamark Environment List, or TIDE. That was the case with Tamerlan Tsarnaev and his mother.

Chatter between them about extreme Islam, Tamerlan's trip to Russia, even an FBI investigation was not enough to move them to a higher level of surveillance.

"Not everybody who travels is going to commit a terrorist act and not everyone who talks about jihad will necessarily commit a terrorist act. And one of the problems with these lists is being able to differentiate chatter from an actual threat," said Marcus.

"In hindsight, this is a guy who had a really terrible intention, and we all wish he was placed on a higher list."

Many of the people on TIDE aren't terrorists. But the Boston marathon bombings prove dangerous terrorists are also among them, too easily overlooked or not on the radar at all.

"One wouldn't have expected that the younger brother would have been involved in terrorism. The clues weren't there. It's just another example of the difficulty in being able to link human behavior to the kinds of acts we saw here in Boston," said Marcus.

The TIDE list has also been criticized for confusing names and redundancies. Even the late Sen. Edward Kennedy was on the no-fly list because his name was similar to the alias of a suspected terrorist.

In the aftermath of the Boston bombings, the federal government will be most focused on making sure those on watch lists who mean to terrorize are stopped.

CALLING ON SYRIA TO RELEASE HIM. POST 9/11, TERROR WATCH LISTS WERE CREATED TO TARGET AND TRACT ACTUAL TERRORIST. BUT THOSE LISTS DID NOT SLOW DOWN SUSPECTED MARATHON BOMBER TAMERLAN TSARNAEV AND HIS MOTHER. WE ASKED THE EXPERTS HOW THAT CAN HAPPEN. BOSTON IS NOT ALONE IN THE VULNERABILITY TO TERRORIST ATTACKS ON AMERICAN SOIL. RARELY A TREMENDOUS VULNERABILITY ABOUT OUR CITY CENTER. ANALYST TONY COOPER IS TALKING ABOUT A THWARTED ATTEMPT TO BECOME A DALLAS HIGH-RISE BACK IN 2009. THIS MAN WAS ON THE HIGH-LEVEL TERROR WATCH LIST UNDER CONSTANT SURVEILLANCE BY THE FBI. ANOTHER TERRORIST, MOHAMED MOHAMED WAS ON THE G. HE OF'S NO-FLY LIST. IN 2010, HE PLANNED TO DETONATE A BOMB IN PORTLAND, OR FON. NEITHER MEN KNEW THEY WERE PART OF A UNDERCOVER STING OPERATION. THE EXPLOSIVES WERE DUMMY. SOMEONE WAS ASSIGNED BY THE FBI TO START WATCHING THEM. VNTLY, SOMEBODY OFFERED THEM EXPLOSIVES. THIS WAS SOMEBODY WORKING WITH LAW ENFORCEMENT TO ACTUALLY PRESS A BUTTON TO DETONATE THE EXPLOSIVES SO THEY WERE PROSECUTED FOR TERRORIST ACT. BOTH CASES POINT TO HOW TERROR WATCH LIST CAN STOP AN ATICK AND TAKE TERRORISTS OFF THE STREET, BUT WHICH LIST THEY ARE ON DETERMINES THE LEVEL OF SURVEILLANCE BY THE GOVERNMENT. SOPHIE F THEY ARE TALKING TO KNOWN BAD GUYS. IF THEY ARE RECEIVING MONEY, THROUGH ROOTS USED BY TERRORISTS. THOSE ARE ALL CLUES. THEY ARE ON HIGH WATCH LIST. THEN, THROUGH LEGAL MEANS, IT IS POSSIBLE TO-ER TO THEIR E-MAILS, TO LISTEN IN ON THEIR TELEPHONE CONVERSATIONS, AND DETERMINE THE PLANS ARE. BUT MOST PEOPLE, SOME HALF MILLION, ARE CONSIDERED LOW RISK AND PLACED ON THE DATA MARK ENVIRONMENT LIST. THAT WAS THE CASE WITH TAMERLAN TSARNAEV AND HIS MOTHER. CHATTER BETWEEN THEM ABOUT EXTREME IS LIMB, THE TRIP TO RUSSIA, EVEN AN FBI INVESTIGATION WAS NOT ENOUGH TO MOVE THEM TO A HIGHER LEVEL OF SURVEILLANCE. NOT EVERYBODY WHO TRAVELS IS GOING TO COMMIT A TERRORIST ACT. NOT EVERYONE WHO TALKS ABOUT JIHAD WILL NECESSARILY COMMIT A TERRORIST ACT. THE PROBLEM FOR THE LIST IS BEING ABLE TO DISTINGUISH CHATTER FROM THE ACTUAL THREAT. IN HINDSIGHT, THIS IS A GUY WHO HAD REALLY, REALLY AWFUL INTENTION AND WE ALL WISH HE WAS PLACED ON HIGHER LIST. MAN HI OF THE PEOPLE UNARE NOT TERRORISTS AT ALL. THE BOSTON MARATHON BOMBINGS PROVED DANGEROUS TERRORISTS ARE AMONG THEM. TOO EASILY OVERLOOKED OR NOT ON THE RADAR AT ALL. ONE WOULDN'T HAVE EXPECTED NA THE YOUNGER BROTH WEIR HAVE BEEN INVOLVED. THE CLUE WAS THERE. IT IS JUST ANOTHER EXAMPLE OF THE DIFFICULTY IN BEING ABLE TO LINK HUMAN BEHAVIOR TO THE KIND OF ACTS WE SAW HERE IN BOSTON. THE LIST HAS BEEN CRITICIZE FOR CONFUSING NAMES. ON THE LATE SENATOR EDWARD KENNEDY LAND ON THE NO FLY